Americium is the only synthetic (man-made) element that can be purchased at Walmart. It does not exist in nature and all of it that exists in the world, and at Walmart, was creating in nuclear reactors and extracted from spent fuel rods during reprocessing. Despite not having existed anywhere on the planet fifty years ago, today it can be found in the majority of homes, in the form of a tiny button deep inside your smoke detector: It is used in the ionization chamber that is the active sensing element in most smoke detectors.

Smoke detector element.
I'm told it's illegal to disassemble a smoke detector and remove the tiny radioactive dot that is contained in every ionization detector. But I did anyway, using an old one in May 2002. Considering that probably many thousands of these are disposed of (also technically illegally) in the garbage every year, I'm hardly the only one to mishandle the dots. My americium dot is now contained in a glued-shut box with a glass top that is harder to disassemble than the smoke detector was, and then inside a lead cup with a lid, just so our personnel manager doesn't freak out. (It was my first radioactive sample, but has since been vastly surpassed in radioactivity by the Fiestaware bowl.)

This article by Ken Silverstein (first published as "The Radioactive Boy Scout" in Harper's Magazine, November 1998) describes the amazing case of a teenage boy who did incredible (and incredibly dangerous) things with smoke detector americium. (The story is now available as a book by the same author.)

The sound is from the Geiger counter: I think most of the radioactivity is shielded by the glass cover glued in place over this sample, so it's not really representative (an unshielded one registers about 2000 counts per minute).

I chose this sample to represent its element in my Photographic Periodic Table Poster. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.

Intact smoke detector.
This is the outer casing of the ionization chamber inside an ordinary smoke detector, along with the text from the label on the back indicating its Americium content. Even though it's not radioactive on the outside, I've got it stored in the Hot Box.Source:Hardware StoreContributor:Theodore GrayAcquired:18 August, 2002Price: $10/smoke detectorSize: 1.25"Purity: >90%

Sample from the Everest Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (except gases) weigh about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.

Radioactive elements like this one are represented in this particular set by a non-radioactive dummy powder, which doesn't look anything like the real element. (In this case a sample of the pure element isn't really practical anyway.)

Modern spinthariscope.
The spinthariscope was a popular amusement at academic parties in the early 1900s, or so I am told. Recently they have become available again on eBay and maybe other places. Antique ones used radium, but modern ones sensibly use americium, which is much safer and is easily available in any smoke detector.

After about 10 minutes of rigorous dark adaption (absolutely no light allowed) you can clearly see a swarming glow on the zinc sulfide screen, each flash representing the decay of a single atom of americium and the subsequent striking of the released alpha particle into the screen. It's amazing.